The subject invention relates to a reliable connector for bonding shields of telephone cables to ground and in particular for the polyethylene coated aluminum shield adhering to and underlying a polyethylene jacket.
The most commonly used devices for providing shield bonding consist of a connector composed of two or more parts assembled together by stud and a nut. The lower sections of the connector, incorporating the stud, are placed in contact with the shield under the cable outer jacket. The other part of the connector rests on top of the cable jacket, whereupon the nut is threaded onto the stud, electrical contact is established between the shield through the lower sections then through the threaded stud when the nut is tightened. This technique is operative to provide shield bonding but it involves many problems.
The electrical contact resistance has been found to increase substantially with time and, as a result, the telephone operating companies have experienced noisy lines.
The high increase in contact resistance with commercially available connectors is attributed to the loss of contact between the lower section and the aluminum shield as a result of aluminum oxidation at the contact points. Aluminum, as well as the polyethylene cable jacket, which is normally low density type, have the tendency to cold flow or creep under sustained load, and, in addition, the dimensional stability of the jacket is very sensitive to temperature fluctuation. Therefore, with time, relocation of the initially applied pressure at the contact points takes place and aluminum oxide forms which is non-conductive and consequently results in increasing the electrical contact resistance.
The main problem, of course, is the bonded composite cable sheath. The coating on the aluminum shield was initially designed to eliminate corrosion problems. Further sheath modification consisted of bonding the shield to the polyethylene cable jacket. The latter improved the mechanical characteristics of the sheath and reduced substantially the moisture permeation into the cable core. The laminated composite sheath, however, created problems in bonding such sheath to ground, or in establishing electrical shield continuity at splice points, since high adhesion between the shield and the jacket made separation of the two difficult, if not practically impossible. This, in turn, created a necessity for electrical bonding of the composite sheath.
The invention described in this specification has built-in features which compensate for creep strain and, in addition, prevent oxidation of the aluminum at the contact points. A connector, according to the present invention, provides and maintains a stable low resistance electrical connection to a metallic shield when applied over the composite sheath of the plastic telephone cable. The present invention overcomes disadvantages of the prior art by providing means by which the contact resistance remains low and stable.
Other objects, features of the invention will appear or be pointed out as the description proceeds.